1. Field of the Invention
The present invention relates to servowriters. More particularly, the present invention relates to a servowriter employing a method of unlatching an actuator arm, in a head disk assembly, using a voice coil motor (VCM) voltage limiting circuit to limit actuator arm velocity.
2. Description of the Prior Art
A huge market exists for hard disk drives for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development, at great annual cost, to design and develop innovative hard disk drives to meet increasingly demanding customer requirements.
In hard disk drives, data is stored on magnetic media disks in concentric data tracks, which are divided into groups of data sectors. Servo information is recorded in radially continuous narrow wedges between the groups of data sectors. A head disk assembly of a disk drive includes an actuator assembly having a voice coil motor (VCM), an actuator arm extending from the VCM, and a transducer head disposed at the end of the actuator arm. The VCM includes a coil moving in proximity to a permanent magnet. The VCM swings the actuator arm and the transducer heads back and forth over the disk to access target data tracks on the disk surface.
During a latch mode, the transducer head is parked away from the data tracks to protect the transducer head and the disk surface, and a latch, such as a magnetic latch, typically restrains the actuator arm in place in the head disk assembly. During an unlatch mode, the VCM is controlled to overcome the force of the magnetic latch to move the actuator arm away from the latch, in what is referred to as unlatching the actuator arm, to position the transducer head over the data area of the disk surface. The actuator arm must be unlatched so that the transducer head can move radially across the disk surface while the head disk assembly is connected in either the disk drive or a servowriter. A problem exits because the amount of current supplied to the VCM to break free of the magnetic latch causes the actuator arm to move away from the latch at a variable actuator arm velocity toward an outer diameter crash stop in the head disk assembly or a push pin in the servo writer. The actuator arm must be slowed to an actuator arm velocity which will not cause damage to the transducer head or disk surface during impact of the actuator arm against the outer diameter crash stop or the push pin.
There exits substantial competitive pressure to develop mass-market hard disk drives with more robust designs that are less sensitive to operator handling and external vibrations. In particular, if the actuator arm unlatches from the latch and the transducer head lands into the data area of the disk, the transducer head and/or the disk surface can be severely damaged. It is known to provide stronger latches to resist the actuator arm coming off the latch when the disk drive is in the latch mode, such as during the manufacturing process or user handling of the disk drive. However, the increasing strength of magnetic latches and other latches makes it more difficult to unlatch the actuator arm, and higher unlatching currents need to be applied to the VCM in order to unlatch the actuator arm The increased unlatching currents result in a larger variability of possible actuator arm velocities after the actuator arm moves away from the latch, which increases the probability of the actuator arm hitting the outer diameter crash stop or push pin at too fast of an actuator arm velocity.
When the head disk assembly is placed in the servowriter for writing servo information on the disks during the manufacturing process, a first series of current pulses are applied to the VCM in order to move the actuator arm away from the magnetic latch. A second series of current pulses are then applied in the reverse direction to reduce the actuator arm velocity. The amplitude and widths of the second series of current pulses can be ascertained by empirical analysis on a disk drive product. An example current pulse sequence comprises an unlatch pulse period, a coast period, and a brake pulse period. The unlatch pulse period typically applies positive current to the VCM for a predetermined period of time. The coast period is a predetermined period of time where zero current is applied to the VCM. The brake pulse period typically applies negative current to the VCM for a predetermined period of time. For such a current pulse sequence, the predetermined time periods and the amount of current applied to the VCM during the unlatch pulse period and during the brake pulse period must have sufficient margins to allow for variations in the VCM and latch assembly of the particular disk drive. Moreover, if the negative current applied to the VCM to slow the actuator arm during the brake pulse period is too high or if the negative current is applied for too long of time period, the actuator arm may return to an inner diameter crash stop and become re-latched in the latch.
When the head disk assembly is connected in the disk drive, a similar current pulse sequence is used to unlatch the actuator arm from the magnetic latch. However, the brake pulse period is typically employed to slow down the actuator arm velocity to a sufficiently slow velocity to permit a servo system in the disk drive to detect the servo information on the disk surface. Once the servo system of the disk drive is able to detect the servo information, the servo system employs conventional closed loop servo control to control the actuator arm velocity and the position of the actuator arm over the disk. If the servo system, however, does not detect the servo information, the actuator arm velocity can not be controlled by the closed loop servo system. If the actuator arm velocity is not controlled by the closed loop servo system, the actuator arm velocity when the actuator arm hits the outer diameter crash stop can be at a level which causes damage to the transducer head and/or the disk.
A disk drive that employs a ramp load design may include a velocity feedback circuit to control the actuator arm velocity. The velocity feedback circuit employs the back electromotive force (BEMF) of the VCM to monitor and control the actuator arm velocity to thereby control the speed at which the transducer head moves away from the ramp and over the disk surface. The velocity feedback circuit includes complex closed loop circuitry to measure the BEMF and compute actuator arm velocity that is used for controlling the amount of current applied to the VCM.
For reasons stated above, there is a need for a circuit or method to control the actuator arm velocity in a cost effective manner in a head disk assembly of a hard disk drive after the actuator arm is released and moves away from the latch.
The invention can be regarded as a method of unlatching an actuator arm from a latch restraining the actuator arm in a head disk assembly connected to a servowriter. The servowriter includes a first node and a second node. The head disk assembly includes a voice coil motor (VCM) coupled to the actuator arm. The VCM includes a coil connected between the first node and the second node. The method includes applying a voltage between the first node and the second node to cause current to flow through the coil in order to move the actuator arm away from the latch at an actuator arm velocity. The method further includes temporarily activating a VCM velocity control signal to enable a VCM voltage limiting circuit connected in parallel with the coil between the first node and the second node. The method includes limiting the voltage applied across the coil to a predetermined VCM voltage level with the enabled VCM voltage limiting circuit in order to limit the actuator arm velocity.
The invention can also be regarded as a servowriter connectable to a head disk assembly that includes an actuator arm, a latch for restraining the actuator arm and a voice coil motor (VCM) coupled to the actuator arm The VCM has a coil. The servowriter includes a first node and a second node. The coil is connected between the first node and the second node. The servowriter includes a VCM driver for applying a voltage between the first node and the second node to cause current to flow through the coil in order to move the actuator arm away from the latch at a variable actuator arm velocity. The servowriter further includes control means for temporarily activating a VCM velocity control signal. The servowriter includes a VCM voltage limiting circuit connected in parallel with the coil between the first node and the second node. The VCM voltage limiting circuit is responsive to the VCM velocity control signal being temporarily activated for limiting the voltage applied across the coil to a predetermined VCM voltage level in order to limit the actuator arm velocity.